The present invention relates to a power transmission device for a four wheel drive vehicle, and particularly to a power transmission device for a four wheel vehicle such as a front engine-front wheel drive vehicle. More precisely, it concerns a differential motion limiting mechanism in a power transmission device for a full-time four wheel drive vehicle which transmits rotation to the front and rear wheels at all times.
Heretofore, there have been proposed various kinds of power transmission devices for four wheel drive vehicles, which are obtained by slightly modifying power transmission devices for transverse front-engine front wheel drive vehicles.
In the above-mentioned conventional devices, as shown in FIG. 3, drive power from an engine transmitted through a hollow shaft 45' to a differential gear carrier (diff-carrier) 41' rotatably supported on shaft sections 42'a, 43'a of both side gears 42', 43' in a center deferential gear 33', is distributed to both side gears 42', 43' from diff-pinion 46' on the diff-carrier 41', the driver power transmitted to the left hand side gear 42' being transmitted through another hollow shaft 47' to a front wheel side differential gear (not shown) and distributed among side gears to be transmitted to front axles on both sides (only the right front axle 31' is shown in FIG. 3.), while the drive power transmitted to the right hand side gear 43' is transmitted through a ring gear mount casing 32' and a rear wheel drive ring gear 35' to a gear 40' and then from a drive pinion shaft 39' through a propeller shaft and a final speed reduction gear (not shown), to be distributed among rear axles on both sides.
It is noted that a center differential gear 33' is indispensable for a full-time four wheel drive vehicle in order to prevent the vehicle from experiencing the phenomenon commonly known as tight cornering brake, which is a braking condition due to a difference in rotational speeds between the front and rear wheels when the vehicle makes a tight turn at low speed on a road surface having a high friction coefficient. On the other hand, it is necessary to provide a differential motion limiting mechanism for stopping or limiting the operation of the differential gear, since the existence of the center differential gear 33' results in a condition where no torque can be transmitted due to the operation of the differential gear, when any one of the wheels of the four wheel drive vehicle loses grip. The conventional differential motion limiting mechanism comprises a sleeve S for engaging and disengaging the left hand side gear 43' against an inner ring 41'a supporting the pinion shaft 46' of the diff-carrier 41', this sleeve S being slidably supported on the shaft section 43'a of the side gear 43' through splines. As shown in the lower half section of FIG. 3, when the sleeve S is retracted into the shaft section 43'a, the center differential gear 33' is in its operating condition, and when it is extended from the shaft section 43'a and its front end engages the inner ring 41'a of the diff-carrier 41 as in a dog clutch, the center differential gear 33 is locked.
In a mechanical differential motion limiting mechanism such as the dog clutch type mentioned above, if the difference in rotational speeds between the side gear 43' and the inner ring 41'a is large during abrupt turns, it is difficult to engage the sleeve S with the inner ring 41'a, and also if a large surface pressure due to torque acts upon the engaged tooth surfaces of the sleeve S with inner ring 41'a, it is difficult to disengage the sleeve S from the inner ring 41'a. Hence, it is impossible to engage and disengage the differential motion limiting mechanism smoothly and swiftly.
Accordingly, an object of the present invention is to provide a differential motion limiting mechanism for a fulltime four wheel drive vehicle, which can be engaged and disengaged smoothly and swiftly, by use of a hydraulic friction clutch in the differential motion limiting mechanism.